Really weird issue with xls being overwritten.

My app contains 2 .xls workbooks, the main workbook and the database workbook. Users run the main workbook and do.....stuff, which writes data results to the database workbook.

To guard against errors I have the following code in place in case two users try to submit data to the database workbook at the same time:

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The above is called from the macro that tries to write data to the database workbook thusly:

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This works absolutely fine in a test environment, the main workbook will just pause for a few milliseconds when clicking 'Submit' until the database workbook is available. If I open the database workbook manually on another PC then the above code will just sit at an hourglass until I close it on said other PC. No problem.

However, when testing in a larger, slower, environment I've found something very odd.
Very, very rarely (so rarely in fact that it's impossible to replicate), instead of pausing until the database workbook is free instead it gets overwritten by a new, blank workbook with one single test result in cell A1.

And I can't work out why. It might be something to do with a slow network or old, dodgy file server that's causing it to not be able to find the workbook file......

Further testing has discovered that if the database workbook doesn't exist then the main workbook will create a new (blank) one and save it in the original database workbook's location. That I don't get either. Surely, the above code would error in the isfilelocked function and therefore return an = true value and loop?

shameless <PUNT>